Expand all search paths so that using ~ is possible

Hi,

I store the rust source somewhere in my home, but the path to my home varies depending on the machine I am using.
I made it possible to use "~/" in the `search_paths`.

I also added some tests. There's quite a lot of monkey patching but running them is quite simple and it catches syntactic problems so it's better than nothing I hope ;-)

You can run the tests with `python3 tests.py` but this will require either 
* system python 3 with the package `mock` installed (`pip install mock`)
* an active python 3 virtualenv with `mock` installed


Thanks a lot for this plugin, it works really well for me!